Omni-directional antennas are typically used by mobile nodes communicating in a network. A node transmitting signals via an omni-directional antenna produces radio interference in other nodes that may be receiving signals via omni-directional antennas. Onmi-directional antennas do not have directional power gain and have poor LPI (low probability of intercept) properties.
U.S. Pat. No. 6,353,601, issued to Siira on Mar. 5, 2002, discloses a method for space division multiple access (SDMA) wherein nodes are distinguished from one another on the basis of their location, and fixed nodes use directional antenna beams to communicate with mobile nodes. When a fixed node wishes to communicate with a mobile node, it calculates the location of the mobile node and orients its directional transmit beam toward the mobile node. Onmi-directional antennas, however, are used by the mobile nodes. The mobile nodes, thus, do not have directional power gain and can not avoid transmission interference from other nodes.
U.S. Patent Application Publication No. 2001/0003443, disclosed by Velazquez on Jun. 14, 2001, describes a SDMA communication system wherein each node employs directional antenna beams and knowledge of each other's position to form narrow antenna beams to and from each other. The position of a node is determined using geo-location techniques, derived via triangulation between nodes or from a global positioning system (GPS) receiver. The orientations of antenna beams of the nodes in the network are not synchronized, and one node has no knowledge of the antenna orientation of another node.
U.S. Pat. No. 5,701,583, issued to Harbin on Dec. 23, 1997, discloses a communication system including fixed and mobile nodes. The receive antennas of the fixed nodes are continuously scanned in the horizontal plane, until such time as a synchronization signal transmitted from a mobile node is received by a fixed node. Upon detection of the synchronization signal, the fixed node stops its scanning until such time as message information can be obtained from the mobile node. After receiving the information, the fixed node resumes scanning of its receive antenna. Although Harbin discloses scanning of receive antennas, there is no suggestion of synchronizing the receive antennas of one fixed node with another fixed node. Thus, one node in the communication system has no knowledge of the antenna orientation of another node in the system.